Workup for hyponatremia

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hyponatremia diagnostic workup urine osmolality urine sodium guideline European clinical practice guideline

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https://pubmed.ncbi.nlm.nih.gov/24569125

Workup for hyponatremia

Hyponatremia = serum Na <135 mmol/L. First establish severity and acuity. This approach applies to adults and should be adapted to the clinical setting.

1. Stabilize first: identify emergencies

Assess neurologic and respiratory status immediately.

Severe symptoms: seizure, coma, marked confusion, repeated vomiting, respiratory distress, or signs of raised intracranial pressure.

Obtain serum sodium urgently, but do not delay emergency treatment for severe symptomatic hyponatremia.
Determine whether onset is acute, generally <48 hours, or chronic/unknown.
Early nephrology/critical-care involvement is appropriate for severe, rapidly falling, or unexplained cases.

Rapid overcorrection can cause osmotic demyelination, so sodium monitoring and treatment need close supervision. The European hyponatremia guideline emphasizes a structured diagnostic and treatment approach.

2. Confirm the result and assess tonicity

Order:

Repeat BMP/CMP: Na, K, Cl, HCO3, BUN, creatinine, glucose
Measured serum osmolality
Consider serum lipids and total protein if pseudohyponatremia is suspected
Serum ethanol or other osmoles if clinically relevant

Interpret serum osmolality

Finding	Interpretation	Typical causes
Low serum osmolality (<275 mOsm/kg)	True hypotonic hyponatremia	Most cases
Normal serum osmolality	Isotonic or pseudohyponatremia	Marked hypertriglyceridemia, paraproteinemia, laboratory artifact
High serum osmolality	Hypertonic hyponatremia	Hyperglycemia, mannitol, radiocontrast, other effective osmoles

For hyperglycemia, calculate a corrected sodium. A commonly used approximation is:

[ \text{Corrected Na} = \text{Measured Na} + 1.6\text{ to }2.4 \times \frac{(\text{glucose}-100)}{100} ]

Use the laboratory's units and local convention.

3. If hypotonic: obtain urine studies before major fluid therapy when possible

The key initial tests are:

Spot urine osmolality
Spot urine sodium
Often urine potassium, urea, and creatinine
Strict intake/output record and daily weight

A. Urine osmolality

Urine osmolality	Meaning	Key possibilities
≤100 mOsm/kg	ADH appropriately suppressed, maximally dilute urine	Primary polydipsia, low-solute intake such as beer potomania or “tea-and-toast” diet, recovery phase of hyponatremia
>100 mOsm/kg	ADH active	Hypovolemia, SIADH, heart failure, cirrhosis, adrenal insufficiency, nausea/pain, medications, hypothyroidism, renal disease

In a hyponatremic patient, urine osmolality >100 mOsm/kg indicates a vasopressin-dependent process. Frameworks for Internal Medicine, p. 509.

B. Urine sodium, interpreted with volume status

Urine Na	Likely interpretation in hypotonic hyponatremia
<20-30 mmol/L	Kidney is conserving sodium: reduced effective arterial volume, often vomiting/diarrhea, third-spacing, heart failure, cirrhosis, nephrotic syndrome
≥30 mmol/L	Renal sodium loss or euvolemic state: SIADH, diuretics, adrenal insufficiency, renal salt wasting, kidney disease

Important caveat: urine sodium may be falsely high with recent diuretic use, chronic kidney disease, adrenal insufficiency, or after IV saline. In patients taking diuretics, fractional excretion of uric acid or urea can sometimes help, but interpretation should be individualized.

4. Assess extracellular-fluid volume status

History and examination should classify the patient as hypovolemic, euvolemic, or hypervolemic, while recognizing that bedside volume assessment is imperfect.

Clinical category	Clues	Common causes
Hypovolemic	Orthostasis, tachycardia, dry mucosa, low JVP, weight loss	GI losses, diuretics, adrenal insufficiency, renal/cerebral salt loss
Euvolemic	No clear edema or depletion	SIADH, glucocorticoid deficiency, hypothyroidism, primary polydipsia, low-solute intake
Hypervolemic	Edema, ascites, elevated JVP, pulmonary congestion	Heart failure, cirrhosis, nephrotic syndrome, advanced kidney failure

A history of nausea, pain, recent surgery, pulmonary/CNS disease, malignancy, poor intake, alcohol use, and fluid intake is particularly useful. Brenner and Rector's The Kidney, p. 1903.

5. Look systematically for causes

Medication review

Ask specifically about:

Thiazide diuretics
SSRIs/SNRIs, antipsychotics, carbamazepine, oxcarbazepine, valproate
Desmopressin
Opioids, NSAIDs
Cyclophosphamide, vincristine, cisplatin and other chemotherapy
MDMA and other recreational drugs

Endocrine tests

In unexplained or apparently euvolemic hypotonic hyponatremia, obtain:

Morning serum cortisol, often with ACTH stimulation testing if indicated
TSH and free T4
Consider ACTH, renin, aldosterone when primary adrenal insufficiency or mineralocorticoid deficiency is suspected

Do not label a patient as having SIADH until adrenal insufficiency and clinically meaningful hypothyroidism have been excluded.

Organ-directed testing as indicated

CBC, liver panel, albumin
BNP, ECG, chest radiograph, echocardiography if heart failure suspected
Urinalysis, urine protein assessment, renal ultrasound if kidney disease suspected
Chest imaging for pulmonary disease or malignancy
CNS imaging only if neurologic signs, head injury, stroke, infection, or malignancy suggests a CNS process

6. Diagnose SIADH only after exclusions

Typical SIADH features:

Hypotonic hyponatremia
Inappropriately concentrated urine: urine osmolality >100 mOsm/kg
Urine Na usually >30 mmol/L with normal salt intake
Clinically euvolemic
No adrenal, thyroid, renal, hepatic, or cardiac explanation
No recent diuretic effect
Low serum uric acid and low BUN can support the diagnosis

SIADH causes include pulmonary disease, CNS disease, malignancy, postoperative state, nausea/pain, and drugs. In cancer, small-cell lung cancer is a classic association. Brenner and Rector's The Kidney, p. 1903.

Practical order set

Repeat BMP/CMP, glucose, Mg, phosphate, BUN/Cr
Measured serum osmolality
Spot urine osmolality, urine Na, urine K, urine creatinine
CBC, urinalysis, liver panel, albumin
TSH/free T4
Morning cortisol, with confirmatory adrenal testing if needed
Medication and fluid-intake review
ECG and targeted cardiac/hepatic/renal/pulmonary/CNS evaluation based on findings
Serial serum Na and urine output in hospitalized or symptomatic patients

Core sequence:
Confirm hypotonicity → urine osmolality → urine sodium + volume status → exclude endocrine/drug causes → targeted search for SIADH or systemic disease.

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